Watersheds are different, and it's a bit risky to draw on national averages as if they represented individual conditions. What we have heard about the blue-green algae problem is that in areas on the Canadian Shield where agriculture is not a major factor, we still see this enrichment. In those particular areas, or those particular lakes, it's certain that the contribution from these detergents is higher.
On June 4th, 2008. See this statement in context.